A frying oil having a high heat stability is known from JP 2 214 799-A. According to this Japanese document, the oil has a low increase in acid value and a low degradation in colour and taste when heated.
The oil is obtained by purifying crude palm oil to provide purified palm oil, fractionating the oil and treating the intermediate fraction with alkali. The IV of the intermediate fraction is 29-46. The treatment with alkali (2-20% NaOH in amounts of 1-5 wt.% at 40-95.degree. C.) is followed by a water wash and a filtering step (activated clay) and finally by a steam deodorization.
Although the properties of this product are acceptable, it does not meet the very high standards for a high-stability oil (=HSO). An HSO should display a RIP (=Rancimat Induction Period) of at least 35 hours (at 120.degree. C.), measured according to the method described in Fette, Seifen und Anstrichmittel, 88, Nr. 2, 1986, pp. 53-56.
Although triglyceride compositions (e.g. Durkex-500.RTM., cf. US Pat. No. 4,748,041) are known that display RIP values of about 35 hours (at 120.degree. C.) (i.e. AOM values of about 350, cf. column 4, lines 5-6), these triglycerides still contain appreciable amounts of solids at ambient temperatures (N.sub.1O = 20, N.sub.20 &lt;10). The iodine value of these oils is at most 81. These triglycerides therefore have the disadvantage that they are not pumpable and/or sprayable at ambient temperatures. Moreover, the colour and colour stability of these oils are not optimal.
A fraction of Durkex-500 (=Durkex-HSO.RTM.) is also mentioned in this patent. This fraction has an I.V. of 80; however, an emulsifier needs to be added in order to make a filtered product cloudless at 10.degree. C. (cf. column 5, lines 1-18).
From JP-55 069 696-A another highly stable fat is known that can be used for frying and spraying. This product is obtained from hardened palm oil. The fat has a high content of trans-acids and displays an AOM value of more than 200 (so, RIP of about 20-25). It is not clear what the solid fat indexes of this fat are at ambient temperature, but, considering the high amounts of trans-acids, they are expected to be rather high.
From GB 867,615 improved soft oils (see page 8) are known that display AOM values of at least 200 hours. According to the Table on page, a maximum AOM of 288 hours can be achieved, which is too low for a high-quality HSO. The reason for such an AOM value is probably that too high amounts of C.sub.18:2 are present in the oils.